1. Field of the Invention
This invention relates to nuclear magnetic resonance (NMR) apparatus. More specifically, this invention relates to radio frequency (RF) coils useful with such apparatus for transmitting and/or receiving RF signals.
2. Description of the Prior Art
In the past, the NMR phenomenon has been utilized by structural chemists to study, in vitro, the molecular structure of organic molecules. Typically, NMR spectrometers utilized for this purpose were designed to accommodate relatively small samples of the substance to be studied. More recently, however, NMR has been developed into an imaging modality utilized to obtain images of anatomical features of live human subjects, for example, the orbits, TMJ, neck, spine, heart and extremities. Such images depicting parameters associated with nuclear spins (typically hydrogen protons associated with water in tissue) may be of medical diagnostic value in determining the state of health of tissue in the region examined. NMR techniques have also been extended to in vivo spectroscopy of such elements as phosphorus and carbon, for example, providing researchers with the tools, for the first time, to study chemical processes in a living organism. The use of NMR to produce images and spectroscopic studies of the human body has necessitated the use of specifically designed system components, such as the magnet, gradient and RF coils.
By way of background, the nuclear magnetic resonance on occurs in atomic nuclei having an odd number of protons and/or neutrons. Due to the spin of the protons and neutrons, each such nucleus exhibits a magnetic moment, such that, when a sample composed of such nuclei is placed in a static, homogeneous magnetic field, B.sub.o, a greater number of nuclear-magnetic moments align with the field to produce a net macroscopic magnetization M in the direction of the field. Under the influence of the magnetic field B.sub.o, the magnetic moments precess about the axis of the field at a frequency which is dependent on the strength of the applied magnetic field and on the characteristics of the nuclei. The angular precession frequency, .omega., also referred to as the Larmor Frequency, is given by the equation .omega.=.gamma. B, in which .gamma. is the gyromagnetic ratio (which is constant for each NMR isotope) and wherein B is the magnetic field (B.sub.o plus other fields) acting upon the nuclear spins. It will be thus apparent that the resonant frequency is dependent on the strength of the magnetic field in which the sample is positioned.
The orientation of magnetization M, normally directed along the magnetic field B.sub.o, may be perturbed by the application of magnetic fields oscillating at or near the Larmor frequency. Typically, such magnetic fields designated B.sub.1 are applied orthogonal to the direction of magnetization M by means of radio-frequency pulses through a coil connected to radio-frequency-transmitting apparatus. Magnetization M rotates about the direction of the B.sub.1 field. In NMR, it is typically desired to apply RF pulses of sufficient magnitude and duration to rotate magnetization M into a plane perpendicular to the direction of the B.sub.o field. This plane is commonly referred to as the transverse plane. Upon cessation of the RF excitation, the nuclear moments rotated into the transverse plane begin to realign with the B.sub.o field by a variety of physical processes. During this realignment process, the nuclear moments emit radio-frequency signals, termed the NMR signals, which are characteristic of the magnetic field and of the particular chemical environment in which the nuclei are situated. The same or a second RF coil may be used to receive the signals emitted from the nuclei. In NMR imaging applications, the NMR signals are observed in the presence of magnetic-field gradients which are utilized to encode spatial information into the NRM signal. This information is later used to reconstruct images of the object studied in a manner well known to those skilled in the art.
In performing NMR studies, it has been found advantageous to use a high homogeneous magnetic field B.sub.o. This is desirable in the case of proton imaging to improve the signal-to-noise ratio of the NMR signals. In the case of spectroscopy, however, this is a necessity, since some of the chemical species studied (e.g., phosphorus and carbon) are relatively scarce in the body, so that a high magnetic field is necessary in order to detect usable signals. As is evident from the Larmor equation, the increase in magnetic field B is accompanied by a corresponding increase in .omega. and, hence, in the resonant frequency of the transmitter and receiver coils. This complicates the design of RF coils. One source of difficulty is that the RF field generated by the coil must be homogeneous over the body region to be studied. Another complication arises from the intrinsic distributed inductance and capacitance in such large coils which limit the highest frequency at which the coil can be made to resonate.
For high resolution magnetic resonance imaging (MRI) of the human body, it is desirable to optimize the signal to noise ratio (SNR) of the received signals. This requires that the size and shape of the receiver surface coil provide an optimum filling factor given the organ of interest. Small RF coils provide larger B.sub.1 fields per unit current, while presenting a smaller surface area to the load. The reduction in the surface area of the load results in less degradation of the coil circuit Q, reducing coil losses.
As previously noted, popular regions of interest for surface coils include the orbits, TMJ, neck, spine, heart and extremities. A number of different geometries with varying degrees of performance have been proposed to optimize SNR for each of these regions of interest. The spinal area presents a particular challenge to coil designers because the clinician would like to vary the field of view of the area as well as survey the entire spine as well as for other body areas in a single acquisition and at the same time obtain the highest possible SNR for high in-plane resolution from thin slices. The result is that surface coils for the spine are often a compromise between field of view and high performance.
Attempts have been made to solve this problem by constructing coils which either can be repositioned or which have sets of elements which can be switched into place to change the field of view. Repositioning devices can be very effective and easy to use because there is no need to reposition the patient. The coil simply moves in a space created between the patient and the table, but this requires a machine of special construction. The use of multi-element coils, however, is limited by the number of sections which can be effectively switched. In addition, such switching requires PIN diodes at each junction, which may reduce the efficiency of the design by reducing the coil Q.
Another approach to the problem is to construct an array of coils which have no mutual inductance and to couple each coil to a separate pre-amplifier, phase detector and combiner (14). This approach is effective, however, it requires additional expensive electronics not available to most sites without extensive redesign by the manufacturer.
Probes or RF coils are also typically constructed of fixed dimensions and designed for the region to be examined. Inasmuch as the same region of interest may vary greatly in size for different patients such coils are limited in their use and application. Also, it is desirable to obtain images of a broad scope for some regions and then image a smaller, more specific area which cannot be done practically with a fixed dimension coil.
It is therefore an object of the invention to provide a flexible, efficient, low-cost solution to the problem of varying the field of view.
Another object of the invention is to provide an NMR RF coil that is variable in size, but which remains electrically stable during adjustment.
A further object of the invention is to provide an NMR RF coil which can be readily and efficiently tuned to allow variation of the field of view of an NMR image.